Pluto was at first considered to be a planet. Our 9th planet, but with the time the astronomers discovered more similar sized objects, as Pluto is tinier even than our Moon, in what is now called the Kuiper Belt. That Kuiper Belt is right where Pluto’s orbit is.
This is why, after some debate, the IAU decided to demote Pluto and name it a Dwarf Planet. I would have preferred it to be called an asteroid as it is smaller than our Moon, really.